Centrifugal Compressors

Apart from the minimum theoretical energy for compression, energy consumption also results from:

• gas friction in impellers and diffusers;

• bearing and seal friction;

• labyrinth leakage both between wheels and into the seal system;

• balance piston leakage from discharge to suction;

• leaking anti-surge valves and/or poorly calibrated anti-surge protection. Minimizing these losses can be achieved as follows:

It is desirable that the compressor' s interior surfaces with a positive pressure gradient are highly polished to reduce friction. Note that polishing areas with a negative pressure gradient may risk premature onset of surge (discussed below) -consider the top surface of an aircraft' s wing which has a matt finish (negative pressure above the wing) compared to the leading edge which is polished (positive pressure). An alternative to polishing is to coat the surfaces of the gas flow path (rotor and stator) and a number of proprietary products are available.

Energy loss due to oil seals can be considerable. The oil supply must be pumped to above the compressor's suction pressure and the return sweet oil must be cooled since the tight clearances in an oil seal result in heating due to the high shear rate. The use of dry gas seals reduces these losses to near zero. Dry seals are not only far cheaper than seal oil systems, but many users have found that retrofitting is justified.

The vast majority of compressors use oil-lubricated bearings. The associated energy loss can be seen in the power required to pump the oil and the amount of heat removed in the oil cooler. Slowly, magnetic bearings are being used, initially for pipeline compressors in cold remote locations and are now available for most process applications. Unfortunately, these generally cannot be retrofitted to existing compressors since the bearings are physically larger -especially the thrust bearings. For new installations, magnetic bearings plus dry gas seals result in an oil-free compressor. Surprisingly, magnetic bearings control shaft position to a tighter tolerance than oil-lubricated sleeve bearings and can even give active vibration control. The tighter tolerance between the rotor and labyrinths also improves efficiency through reduced internal leakage.

Electromagnet n Á

Compressor shaft

Electromagnet

Position-sensing probe n Á

Compressor shaft

Figure 8.1 Magnetic journal bearing.

Magnetic bearings operate by sensing the distance between the probe(s) and shaft, and adjusting current to the electro magnets to maintain the shaft position. Figure 8.1 illustrates the general principle of operation. If current is increased to all the magnets at the same time, the bearing stiffness is increased. Because there is close to zero friction in magnetic bearings and dry gas seals, the pressure-velocity limit (PV Limit) inherent in oil-lubricated sleeve and thrust bearings disappears, allowing increased speed. Finally, because of the rapid response of the magnets, these bearings can be used for active vibration control.

A traditional labyrinth is simply a series of knife edges set into a ring mounted inside each diaphragm, with the edges facing inwards towards the rotating shaft. Typical clearance is 1.3 to 1.5% of the compressor shaft diameter and so there is always some leakage. Leakage increases if the labyrinths are fouled or worn (such as can happen due to surge or not accelerating quickly through first lateral critical speed). This leakage can be reduced to near zero through retrofitting improved design such as abradable labyrinths where the knife edges are mounted on the shaft and allowed to cut into an abradable ring mounted in the diaphragm.

Some compressor services carry the risk of internal fouling. Rotor and stator coatings can reduce this tendency. It is common to inject wash oil into the suction of compressors that foul (for example, cracked gas compressors), but this wash oil increases power requirements and so should be minimized consistent with achieving the desired result. Since polymer fouling increases with gas temperature, suction cooling and inter-cooling should be maximized.

All dynamic compressors (centrifugal and axial) exhibit instability if the suction volume flow is too low. This is known as surge, resulting in periodic flow reversal. It can be violent for axial compressors, and for both centrifugal and axial there is a risk of thrust bearing damage. The protection consists of ensuring that the suction flow is safely above the surge point over the full operating speed range. Other than air compressors, which avoid surge by venting from the discharge, process compressors take some of the discharge flow and spill it back into the compressor suction, usually via a cooler. Anti-surge protection should ideally not normally require continuous spillback except when the plant is running at low throughput. The surge protection margin should be no greater than 5% above flow at surge. Advanced anti-surge controls are self-calibrating to ensure that spillback flow (which wastes compression energy) is kept to the safe minimum. If the process demand is always small enough to result in anti-surge spillback, then in the short term investigate suction throttling and long term the compressor should be modified.

Low flow, high pressure ratio is more efficiently handled by reciprocating compressors. However, mainly on cost grounds a number of these applications have used centrifugal machines, especially offshore where the weight and vibration of reciprocating compressors is more difficult to accommodate.

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